The invention relates to a method for processing video pictures for display on a display device. More specifically the invention is closely related to a kind of video processing for improving the picture quality of pictures which are displayed on matrix displays like plasma display panels (PDP) or other display devices where the pixel values control the generation of a corresponding number of small lighting pulses on the display.
Although plasma display panels are known for many years, plasma displays are encountering a growing interest from TV manufacturers. Indeed, this technology now makes it possible to achieve flat color panels of large size and with limited depths without any viewing angle constraints. The size of the displays may be much larger than the classical CRT picture tubes would have ever been allowed.
Referring to the latest generation of European TV sets, a lot of work has been made to improve its picture quality. Consequently, there is a strong demand, that a TV set built in a new technology like the plasma display technology has to provide a picture so good or better than the old standard TV technology. On one hand, the plasma display technology gives the possibility of nearly unlimited screen size, also of attractive thickness, but on the other hand, it generates new kinds of artifacts which could damage the picture quality. Most of these artifacts are different from the known artifacts occurring on classical CRT color picture tubes. Already due to this different appearance of the artifacts they are more visible to the viewer since the viewer is used to see the well-known old TV artifacts.
The invention deals with a specific new artefact, which is called xe2x80x9cdynamic false contour effectxe2x80x9d since it corresponds to disturbances of gray levels and colors in the form of an apparition of colored edges in the picture when an observation point on the matrix screen moves. This kind of artefact is enhanced when the image has a smooth gradation like when the skin of a person is being displayed (e. g. displaying of a face or an arm, etc.). In addition, the same problem occurs on static images when observers are shaking their heads and that leads to the conclusion that such a failure depends on the human visual perception and happens on the retina of the eye.
Some approaches have been discussed to compensate for the false contour effect. The false contour effect is directly related to the sub-field organization and the more sub-fields will be used, the better the result is. The term sub-field organization will be explained in greater detail below but for the moment it should be noted that it is a kind of decomposition of the 8-bit gray level in 8 or more lighting sub-periods. An optimization of such a picture encoding will have, indeed, a positive effect on the false contour effect. Nevertheless, the increasing of the sub-field number needs to allocate more time for the addressing periods (since information has to be loaded in the panel for each sub-field) and the complete time available for addressing and lighting is limited (for instance 20 ms/frame for a 50 Hz panel operating in progressive scan mode).
Another approach for the solution of above-mentioned problem is known under the expression xe2x80x9cpulse equalization techniquexe2x80x9d. This technique is a more complex one. It utilizes equalizing pulses which are added or separated from the TV signal when disturbances of gray scales are foreseen. In addition, since the fact that the false contour effect is motion relevant, different pulses for each possible speed are needed. That leads to the need of a big memory storing a number of big look-up tables (LUT) for each speed and there is a need of a motion estimator. Furthermore, since the false contour effect depends on the sub-field organization, the pulses have to be re-calculated for each new sub-field organization. However, the big disadvantage of this technique results from the fact that the equalizing pulses add failures to the picture to compensate for a failure appearing on the eye retina. Additionally, when the motion is increasing in the picture, there is a need to add more pulses to the picture and that leads to conflicts with the picture contents in case of very fast motion.
From the European Patent Application 98114883.6 of the applicant a different approach for reducing the false contour effect is known which will provide very good false contour reduction without any loss of vertical resolution. However, this algorithm which shiftes sub-fields in a direction determined by motion estimation is more complicated and there is a need to use a well adapted motion estimator. The implementation of this solution could take more time and needs more die-size in an IC.
In EP 0874349 (a patent application of THOMSON multimedia) another approach for reducing false contour effect called Bit Line Repeat technique is described. The idea behind this technique is to reduce, for some sub-fields named common sub-fields, the number of lines to be addressed by grouping two consecutive lines together. For the remaining sub-fields called normal sub-fields each line is addressed separately. Nevertheless this technique, causes a slight degradation of the vertical resolution dependent on the picture content and a new kind of noise could be perceived.
The invention aims to improve the bit line repeat technique in order to deliver better picture quality in terms of vertical resolution and noise. It is an object of the present invention to disclose a corresponding method and an apparatus for processing video pictures for display on a display device. This object is achieved by the measures claimed in claims 1 and 8.
While the bit line repeat algorithm is able to correctly encode lots of pixel value combinations of two or more consecutive lines, there are nevertheless some cases in which an error has to be made due to the reduced flexibility in encoding produced by the need to have the same code on common sub-fields. The general idea of the invention is now to put the coding failures on the higher video levels of the two or more pixels being grouped together (see claim 1). With this new method the reduction in vertical resolution and also the noise caused by the bit line repeat algorithm is shifted in a region where it is merely invisible for the viewer.
Advantageously, additional embodiments of the inventive method are disclosed in the respective dependent claims.
In the field of false contour effect compensation the addition of a dithering pattern to a picture brings some benefit. Especially it is positive for improving gray scale portrayal in a plasma picture. Often the value +1 is added to every other pixel in Quincunx form. To adapt the dithering method to bit line repeat technique, the invention proposes a somewhat different dithering pattern for use in combination with bit line repeat algorithm. Here, always the same value is added to the two or more pixels being grouped together in two or more consecutive lines. The resulting dithering pattern also has Quincunx form (see claim 2).
The bit line repeat method can be further improved by the general idea of making an analysis of the pictures in terms of picture content and switching ON or OFF the bit line repeat algorithm depending on the analysis result (see claim 3). E.g., when the picture content analysis reveals too much high vertical transitions in a number of pictures, the bit line repeat algorithm is switched off (see claim 4). This will improve the picture quality a lot in pictures which contain a lot of high vertical frequencies like pictures containing text or graphic with grids, etc. in which the eye will be more focused on these structures than on false contour effects. In fact it will reduce a lot the loss of vertical resolution in case of long critical scenes.
Further improvement is possible by using a motion detector for detecting motion in the picture. The basic idea is to switch off the bit line repeat algorithm when a frame does not contain enough motion (see claim 5). In case where a video sequence has only minor motion in it, no false contour effect will occur and the bit line repeat technique is not necessary.
These improvements can be refined by making a switching control dependent on the number of frames where motion has been detected or the picture content analysis has revealed that normal sub-field coding will bring better results (see claim 6).
The invention consists further in an apparatus for carrying out the inventive method. Advantageous embodiments for such an apparatus are given in claims 8 to 13.